1. Field of the Invention
The present invention relates to a water electrolysis apparatus formed by stacking a plurality of unit cells, the unit cells each including an electrolyte membrane, circular current collectors disposed on the respective opposite sides of the electrolyte membrane, and separators stacked on the current collectors, wherein a water flow field for supplying water is defined between one of the current collectors and one of the separators and a hydrogen flow field for producing hydrogen by electrolyzing the water is defined between the other of the current collectors and the other of the separators.
2. Description of the Related Art
Solid polymer electrolyte fuel cells generate DC electric energy when anodes thereof are supplied with a fuel gas, i.e., a gas mainly composed of hydrogen, e.g., a hydrogen gas, and cathodes thereof are supplied with an oxygen-containing gas, a gas mainly composed of oxygen, e.g., air.
Generally, water electrolysis apparatus are used to generate a hydrogen gas for use as a fuel gas for such solid polymer electrolyte fuel cells. The water electrolysis apparatus employ a solid polymer electrolyte membrane for decomposing water to generate hydrogen (and oxygen). Electrode catalyst layers are disposed on the respective sides of the solid polymer electrolyte membrane, making up a membrane electrode assembly. Current collectors are disposed on the respective opposite sides of the membrane electrode assembly, making up a unit. The unit is essentially similar in structure to the fuel cells described above.
A plurality of such units are stacked, and a voltage is applied across the stack while water is supplied to the current collectors on the anode side. On the anodes of the membrane electrode assemblies, the water is decomposed to produce hydrogen ions (protons). The hydrogen ions move through the solid polymer electrolyte membranes to the cathodes, where the hydrogen ions combine with electrons to generate hydrogen. On the anodes, oxygen generated together with hydrogen is discharged with excess water from the units.
Japanese Laid-Open Patent Publication No. 09-095791 discloses a water electrolysis apparatus of the type described above. As shown in FIG. 11 of the accompanying drawings, the disclosed water electrolysis apparatus includes a cell 2 comprising a solid polymer electrolyte membrane sandwiched between a disk-shaped anode feeder plate 1 and a disk-shaped cathode feeder plate. A plurality of such cells 2 are stacked with separator plates interposed therebetween. The anode feeder plate 1 is fitted in a casing ring 3 forming a casing.
The anode feeder plate 1 has a plurality of parallel grooves 4 defined in a surface thereof that is not held in contact with the solid polymer electrolyte membrane. Each of the grooves 4 serves as a flow field for pure water and also as a flow field for a generated oxygen-containing gas to flow therethrough. The casing ring 3 has a circumferential groove 5 defined in an inner circumferential surface thereof and held in fluid communication with the grooves 4. The casing ring 3 also has three through holes 6a, 6b, 6c defined therein which extend along the direction in which the cells 2 are stacked.
The through hole 6a, which serves to supply pure water, and the circumferential groove 5 are connected to each other by a passage 7a defined in the casing ring 3 therebetween. The through hole 6b, which serves to discharge pure water and an oxygen gas, and the circumferential groove 5 are connected to each other by a passage 7b defined in the casing ring 3 therebetween. The through hole 6c, which serves to discharge a hydrogen gas, is disposed closely to the through hole 6b. Hydrogen which is generated at the cathode feeder plate by electrolysis of water is introduced into the through hole 6c. 
In the above water electrolysis apparatus, pure water flows from the through hole 6a through the passage 7a to the circumferential groove 5, and is then distributed into the grooves 4. At that time, the pure water tends to be distributed preferentially into the central groove 4a which is positioned near the passage 7a. Therefore, the ability to distribute pure water equally into the grooves 4 tends to be lowered, and the distributed pure water flows at widely different rates through the grooves 4, so that the water electrolyzing process is not performed efficiently.
Also, in the above water electrolysis apparatus, oxygen (reactant gas) generated by a reaction, in addition to unreacted water, flows downstream of the grooves 4. Accordingly, a two-phase flow of water and oxygen exists in the passage 7b, and the generated gas results in an abrupt increase of the volume.
Thus, the pressure loss in the passage 7b is larger than that in the passage 7a in which only water exists. Therefore, since an overall pressure loss in the water electrolysis apparatus is large, a high-power and high-performance water pump is required, which is not economical.